Many chemical reactions and physical processes require heating to trigger or advance the reaction or process. Heating is generally applied to a large volume in which the chemical reagents or workpiece is present.
However, localized heating for certain applications is known. For example, laser ablation using focused laser beams for polymer and doped polymer materials has been widely used for the fabrication of microfluidic channels, optical devices such as waveguides, optical switch, optical memory devices, MEMS packaging and many other significant applications. In these applications, laser irradiation introduces photochemical and/or photothermal effects on the polymer, leading to the deformation or decomposition of the polymer material. To extend the range of polymers that may be processed by laser irradiation, dye molecules with strong absorption at certain wavelength ranges are often added to the polymer as photon-thermal energy converters. The thermal energy released by the dye molecules is transferred to the polymer matrix. For most of the existing laser ablation processes, open channels or holes are generated on the polymer or composite substrate materials due to the complete decomposition of polymer molecules upon photoirradiation.
Recently, the photon-thermal energy conversion property of gold nanoparticles has attracted an interest from the scientific community. Gold nanoparticles with core diameters in the tens of nanometers are known to exhibit a strong surface plasmon resonance absorption band at visible light range around 520 nm. The photon capture cross-sections of nanoparticles are four to five orders of magnitude greater than those of photothermal dyes. For example, gold nanospheres with a diameter of 40 nm have a calculated absorption cross-section of 2.93×10−15 m2, which corresponds to a molar absorption coefficient ε of 7.66×109 M−1 cm−1 at a plasmon resonance wavelength maximum λmax of 528 nm. This value is five orders of magnitude higher than the molar extinction coefficient of indocyanine green (ε=1.08×104 M−1 cm−1 at 778 nm).
The photon-thermal energy conversion property of gold nanoparticles is currently being explored for biological applications such as photothermal destruction of cancer cells, bacteria and β-amyloid plaques. For example, a paper entitled “Synergistic enhancement of selective nanophotothermolysis with gold nanoclusters: potential for cancer therapy” by Zharov et al. Lasers Surg Med. 2005; 37 (3):219-26 (ISSN: 0196-8092) discloses an approach that enhances selective photothermolysis of a tumor through laser activation of synergistic phenomena around nanoclusters, which are self-assembled into cancer cells. In vitro verification of this approach was performed by laser pulse irradiation (420-570 nm and 1064 nm; 8-12 nanosecond; 0.1-10 J/cm2) of MDA-MB-231 breast cancer cells targeted with primary antibodies to which 40-nm gold nanoparticles were selectively attached by means of secondary antibodies. The assembly of gold nanoclusters on the cell membrane was found to be accompanied by increased local absorption and red-shifting as compared to cells that did not have nanoclusters. These effects were amplified by a silver-enhancing kit and pre-irradiation of cells with low laser-pulse energy. Finally, a significant increase in laser-induced bubble formation and cancer cell killing was observed using near-IR lasers (1064 nm). A cancer cell antigen was used to provide target specificity for nanoclusters formation making the cancer cells sensitive to laser activation.
It would be advantageous to be able to apply photo-thermal conversion by metal nanoparticles for chemical reactions and physical processing of materials. For these applications, metal nanoparticles are embedded in the chemical reaction media or the materials to be processed in a supported or non-supported format. Regarding chemical reactions, simply running a chemical reaction aided by a light source and metal nanoparticles in solution with the reagents does not provide useful results due to several unsolved issues. One important unsolved issue is stability of the nanoparticles during chemical reaction initiation. Another unsolved issue is how to eliminate the nanoparticles from the reaction mixture or product after the chemical reaction is completed.